The present invention pertains generally to radar and more particularly to tracking radar receivers. Tracking radar receivers are used to generate angle error signals which indicate a deviation of an antenna""s boresight direction from a desired direction. The present invention additionally relates to electronic counter-countermeasures (ECCM), and particularly to ECCM techniques which reduce the vulnerability of a tracking radar receiver to jamming signals which exhibit a frequency substantially equivalent to an image frequency characteristic of the receiver.
An image frequency represents a radio frequency (RF) signal which is separated in frequency from a real RF signal by two times an intermediate frequency (IF) in the direction of a local oscillator frequency. When a receiver employs RF gain prior to down-converting an RF signal into an IF signal, or operates in the presence of standoff jamming, noise which may appear at the image frequency adds to noise which appears at the real frequency in the resulting IF signal. Thus, the signal-to-noise ratio after conversion to IF normally decreases by 3 db over a corresponding signal-to-noise ratio which would be possible without the presence of image noise or standoff jamming.
Receiver designs typically utilize image noise reduction schemes to reduce image noise and thereby increase receiver sensitivity. One scheme uses a conventional image-reject mixer. The conventional image-reject mixer nulls out image frequency signals relative to real frequency signals. An object of a conventional image-reject mixer is to maximize image suppression. Image suppression represents a ratio of amplitudes of an image signal input to a real signal input for identical amplitude outputs. As a practical matter, image suppression of greater than 30 db is obtained and preserved over environmental extremes.
Tracking radar receivers may operate in two modes. In a first mode, tracking commands are generated based on xe2x80x9cskin returnsxe2x80x9d of an object being tracked. In this first mode an increased receiver sensitivity increases the receivers ability to generate accurate tracking commands. Thus, it is desirable to employ an image-reject mixer to increase receiver sensitivity. In a second mode, tracking commands are generated based on a received signal, such as may occur when the object being tracked transmits a jamming signal. In this second mode, receiver sensitivity is relatively unimportant since jamming signals exhibit relatively large amplitudes. Tracking radar receivers may contain a plurality of channels, where each channel contains an image-reject mixer to increase receiver sensitivity. However, this structure makes such a tracking radar receiver vulnerable to a particular type of jamming when operating in the second mode.
If the object being tracked transmits a signal at the receiver""s image frequency, the receiver""s ability to generate tracking commands while operating in a track-on-jam mode may be seriously impaired or totally defeated. The vulnerability occurs because the image-reject mixers are balanced at a null condition at the image frequency. Although signal response of the receiver at the image frequency may be low relative to signal response at a corresponding real frequency, inadequate amplitude and phase tracking occurs between the various receiver channels when image frequency signals are received. For example, the image ratio of one channel might be 40 db while the image ratio of another channel might be only 30 db when overall amplitude response of the channels are equal. In this example, a 10 db amplitude response difference between channels exists at the image frequency. This amplitude difference can cause great deviation in tracking signal characteristics and can even cause polarity reversals. One example of such a polarity reversal might be the generation of an error signal which indicates upward movement when a correct error signal would indicate downward movement. Additionally, the phase relationship of signals output from image-reject mixers at the image frequency might substantially differ between the channels causing a similar result.
Accordingly, one object of the present invention concerns minimizing the degradation of receiver sensitivity to image noise while simultaneously maximizing a tracking radar receiver""s immunity from image frequency jamming.
A further object of the invention concerns providing a partial image-rejection down-converter which can be used to adequately separate image frequencies from real frequencies in a tracking radar receiver while remaining relatively invulnerable to image frequency jamming.
The above and other objects and advantages of the present invention are carried out in one form by an improved receiver for a tracking radar. An antenna having a plurality of antenna elements couples to a combination hybrid which produces sum and difference signals from RF energy received at the antenna elements. The sum signal from the combination hybrid is down-converted to an IF frequency in a first partial image-rejection down-converter. Likewise, the difference signal from the combination hybrid is down-converted to the IF frequency in a second partial image-rejection down-converter. A demodulator produces an error output signal using an input signal from the second partial image-rejection down-converter and a reference signal output from the first partial image-rejection down-converter.